XJ550 backfires, won't start.

Hi everyone. Been trying to get my '82 XJ550 project up and running for the coming season, but I just can't seem to get this ignition system operating properly. I've had the bike running before, but its always had a real tough time getting going. At the moment, it just cranks and cranks. Won't really even light off on starter fluid. Occasionally, I'm getting a backfire/misfire through either the carbs or the exhaust, especially right as I left off the starter button. Was working on it tonight and decided to call it for the day after a 2 ft flame shot out the left exhaust. It has new spark plugs, and I replaced the dead TCI with a Chinese-made on off of ebay. The resistances in the ignition and pickup coils seem fine, but I feel like I've been through everything; I'll try just about anything at this point. Any thoughts/suggestions on how to fix this backfire and get the thing running?

 

This sounds more like a carb issue, you should pull the carbs , and pull the float bowls. Check the main and secondary jets . Clean out the float bowls , since these are Mikuni carbs , not sure if it is like the carbs on my 750 if it is the choke has a small jet in bottom of the bowl . Use some carb cleaner with the red straw and spray it till a jet of fluid comes out it. You also state you replaced the TCI why? You should really stick with stock item . A hard starting situation most times is carb related.

 

Have you checked for spark on each plug?
If you have a gauge, have you checked compression?

 

The carbs are spotless on the inside, though I suppose they could be tuned wrong.

Replaced the TCI because I wasn’t getting spark. I now have spark but was wondering if the aftermarket one maybe made the timing wrong?

Have spark on all cylinders. Compression is around 120psi each.

 

What usually happens if fuel is leaking slightly and the ignition is on it can cause a back fire in the exhaust system, I had a Virago that was doing this , my wife thought I had fired shotgun off. After I replced needle and seat and hot right jets installed it took care of this. So you should check to make sure the Vitron tip of the needle is not grooved .

 

Attempting to start when the starter button is released is a sure sign of a weak battery, and occurs because of low primary voltage to the ignition coils that results in low voltage to the secondary side, or commonly stated as a weak spark

The backfiring is likely a different story and likely means there is work needed on either weak ignition parts or as Jetfixer mentioned the carbs are not functioning as they should. However, if you have a fresh set of plugs (not fouled from excessive fuel) it really should run momentarily with starter fluid if the ignition system / engine is performing satisfactorily

 

I had oreilly’s check the battery; they said it was good. Maybe it’s worth replacing anyways. The one I have is only a year old, but I did buy the cheapest one I could find.

it’s not really even going off with starter fluid, so at this point I’m inclined to say that the issue is with the ignition rather than with the carbs. Don’t have a way to confirm that though I guess. I think maybe I’ll try buying an original Yamaha TCI unit instead of one of those Chinese reproductions and seeing if that fixes it.

 

I've never heard of Chinese TCI can you enlighten us? Did you ever have cause to remove the cams and re-time the valves? Has the valve clearances been set?

 

Here’s the link to the ignition box I ordered:

https://www.ebay.com/itm/4031814579...RxkXKy6SLa&var=&widget_ver=artemis&media=COPY

I did go through and do a shim job on it. One of the first things I checked. I haven’t done anything to the timing of the cams. I guess that could be an issue here.

 

wow who can believe there's even a big enough market to develop such a box.. probably similar unit can be made to suit a bunch of UJMs with some timing tweaks. Seller has good feedback so maybe it's kosher.

 

Well, a rebuilt ignition box got the bike to fire up, so I guess the Chinese one is garbage (or at least the one I got was). There's definitely something wrong with the carbs though; the bike idles at 3k rpm once it warms up. From some reading on the forum there's a good chance that this is due to some bad throttle shaft seals. I'm going to try to fiddle with the tuning a bit first before I go through the effort of taking the carbs off again.

 

A spritz of carb cleaner directly on the ends of the shafts on each side of the carbs will let you know if/which seals are leaking. Works around the intake boots and any vacuum caps/connections too.

 

Carb cleaner didn’t seem to change the rpm at all, so maybe it’s not the seals. I tried to sync the carbs but actually bottomed out the adjustment on carb 3. It seems to start off at a decent idle (maybe 1500, which is still a little high I know) and then shoots up to 4k. Whenever it does this the vacuum gauges on 1, 2, and 4 hardly move, but the vacuum on 3 shoots up. Whatever my issue is, it seems to be with carb 3.

 

Technically carb 3 does not have an adjustment for sync, maybe you are referring to the idle adjustment throttle stop screw?

We don't know the history of the bike, but high vacuum on #3 with a few assumptions just means the throttle plate valve is closed further on that cylinder more than the others - likely from adjusting the throttle stop in an attempt to lower idle. Therefore, the sync screws for the other three would need to be adjusted in a direction that closed their respective throttle plate valves. Closing the other three would lower idle so at the same time you may need to adjust the throttle stop screw to maintain idle

Assumptions - syncing is all about vacuum and vacuum is a function of good even compression across the four cylinders with no vacuum leaks occurring on the intake side. Worn engine parts or valves out of spec contribute to poor / varying compression so a valve check / adjustment is always advised on any bike without known history, and a compression check can be useful to assess if any internal engine damage / wear exits

 

My handbook shows that carb #2 is the reference carb. Sync 1 to 2, then 3 and 4 together. Then sync 3 and 4 as a unit to 2 with the adjustment on #3.

First thing I did with the bike was do a valve adjustment, so I feel pretty good about that. And a compression check shows about 120psi across all cylinders.

 

Use the screw between 3 & 4 to sync 4 to 3, then the screw between 1 & 2 to sync 1 to 2, then the middle screw to sync 1 & 2 to 3 & 4, or at least that is the order for an XJ650

 

Trying to sync again today and still having issues. I’m using the same gauge for all 4 cylinders (taking turns taking measurements) to make sure that that’s not the issue. Retuned 1 to 2 and 4 to 3, but then when tuning 3+4 to 2, the vacuum doesn’t register any change and then randomly shoots up to like 6k rpm from 2. I’m not bottoming out anymore like in my last post, but it’s basically impossible to sync like this. Any thoughts?

 

So after tuning 1 to 2 and 4 to 3 you then place the single vacuum gauge on cylinder #2 and adjust the center balancing screw to match the reading you had on 3 and 4?

I have never tried one gauge before and although it is doable it seems it could get confusing pretty quickly

 

Right, except I’m putting the vacuum gauge on 3 and adjusting to what I had on 1 and 2. 2 is the reference carb on these 550s.

The single gauge just makes it easier in that none of my gauges are measuring the same vacuum even from the same carb and I don’t have to figure out some complicated calibration method for getting good readings from several gauges.